1. Field of the Invention
The invention relates to a method of indirectly connecting two parts and somewhat more particularly to such a method whereby metallic layers are applied under vacuum to the surfaces to be connected and are then brought into contact with one another without ventilation under the maintained vacuum to form the desired bond.
2. Prior Art
German Publication Application DT-OS 2,253,913 discloses a method of indirectly connecting two parts by applying, under vacuum, metallic layers on the surfaces to be connecting and forcing such layers against one another to form a bond. In this method, an acousto-optical building component comprised of, for example, quartz is joined with an electro-mechanical converter, such as a piezo-electric crystal so that the resultant structure comprises an acoustical light deflection system. In order to achieve the desired bond between such parts, two consecutive metallic layers are provided on appropriate surfaces of such parts. A first layer is composed of gold and chromium and the other layer is composed of indium. The metals forming such layers are vaporized from suitable sources within a vacuum chamber and are deposited on the surfaces of the respective parts to be connected, which are then bonded to one another via such layers. In this system, the layer composed of gold and chromium has a thickness of a few thousand Angstrom units and the indium layer is of approximately the same thickness. After such metallic layers have been applied onto the respective surfaces to be joined, the free surfaces of the respective indium layers are brought into contact against one another and compressed under vacuum for a few minutes at a pressure of about 2,800 to 3,500 N/cm.sup.2 (Newtons per square centimeter) so that such indium layers and the parts supporting such layers become interconnected by cold welding. The indirect connection or bond established in this manner is essentially a non-detachable connection, that is, it can only be severed by a break or by a mechanical superstress.
However, the above described prior art method cannot be utilized with all types of materials, particularly when the thickness of the metal layers used to form the indirect bond is relatively small. Nevertheless, it was discovered that as the layer thickness of the metal layers is decreased, the pressure required for cold welding can be reduced (further details regarding the use of relatively thin metal coatings for pressure-free cold welding are disclosed in our companion co-pending application Ser. No. 928,683 filed July 27, 1978 which is incorporated herein by reference). Thus, when metal coatings having a thickness ranging between 0.5 and 55 nm (nanometers) are utilized, practically no pressure is required for forming a cold welded bond. This is of material advantage, particularly with pressure-sensitive materials. However, with metal layer thicknesses below 0.5 nm, it is no longer possible to achieve a firm and durable indirect connection with acousto-optical building components composed of, for example, lead glass or with piezo-ceramic transducers composed of, for example, zirconium titanates. Depending on the thickness of the metal coatings utilized with such materials, either no welded connection at all is obtained or no durable welded connection is obtained. Further, if one of the parts to be joined is made of a metal, in most cases, indirect permanent connections also cannot be produced in this manner.